The present invention relates to a novel radical polymerization process for obtaining block copolymers.
Block polymers are usually prepared by ionic polymerization. This type of polymerization has several drawbacks:
it only allows the polymerization of certain types of non-polar monomers, especially styrene and butadiene,
it requires a particularly pure reaction mixture and temperatures which are often below room temperature so as to minimize parasitic reactions.
The operational constraints are therefore severe.
Radical polymerization has the advantage of being easily carried out without having to comply with excessive purity conditions, and at temperatures greater than or equal to room temperature. During this polymerization, macroradicals, which have a very short lifetime, recombine irreversibly by coupling or dismutation. When the polymerization takes place in the presence of several comonomers, the compositional variation of the mixture is infinitely low compared with the lifetime of the macroradical so that the chains have a random sequence of monomer units and not a block-type sequence.
Consequently, until recently a radical polymerization process allowing block polymers to be obtained did not exist.
Since then, a new radical polymerization process has been developed, namely xe2x80x9ccontrolledxe2x80x9d or xe2x80x9clivingxe2x80x9d radical polymerization. This controlled radical polymerization takes place by the growth, by propagation, of macroradicals.
At the present time, several controlled radical polymerization techniques are known, in which the ends of polymer chains may be reactivated in the form of a radical by homolytic bond (for example, Cxe2x80x94O or C-halogen) scission.
Controlled radical polymerization therefore has the following distinct characteristics:
1. the number of chains is fixed throughout the duration of the reaction,
2. the chains all grow at the same rate, resulting in:
a linear increase in the molecular masses with conversion,
a narrow distribution of masses,
3. the average molecular mass is controlled by the monomer/chain-precursor molar ratio, and
4. the possibility of preparing block copolymers.
The controlled character is even more pronounced when the rate of consumption of the chain precursor is very much greater than the rate of growth of the chains (propagation). There are cases where this is not always true and conditions 1 and 2 are not observed, nevertheless it is always possible to prepare block copolymers.
Several approaches have been described for controlling radical polymerization. The most commonly cited consists in introducing, into the mixture, counter radicals which combine reversibly with the growing macroradicals, such as, for example, nitroxyl radicals (Georges et al., Macromolecules, 26, 2987, (1993)). This technique is characterized by high temperatures for labilizing the Cxe2x80x94O bond.
Another method, called Atom Transfer Radical Polymerization, makes use of transition metal salts combined with organic ligands and an initiator generally consisting of an organic halide; control of the polymerization is made possible by the reversible activation of the C-halogen bond (K. Matyjaszewski, PCT WO 96/30421). One drawback with this polymerization is that it requires a stoichiometric quantity of metal per chain precursor.
Otsu (Otsu et al., Makromol. Chem. Rapid Comm., 3, 127-132, (1982), Otsu et al. ibid, 3, 123-140, (1982), Otsu et al., Polymer Bull., 7, 45, (1984), ibid, 11, 135, (1984), Otsu et al, J. macromol. Sci. Chem., A21, 961, (1984) and Otsu et al., Macromolecules, 19, 2087, (1989)) has shown that certain organic sulphides, particularly dithiocarbamates, allowed chains to be grown in a controlled manner under UV irradiation, according to the principle: 
The principle relies on the photolysis of the Cxe2x80x94S bond, which regenerates the carbon macroradical, on the one hand, and the dithiocarbamyl radical, on the other hand. The controlled character of the reaction is due to the reversibility of the Cxe2x80x94S bond under UV irradiation. It is thus possible to obtain block copolymers. On the other hand, the rate of exchange in propagating species and xe2x80x9cdormantxe2x80x9d species of reaction 1 above is not very large compared with the rate of propagation, this having the consequence of generating relatively broad molecular mass distributions. Thus, the polydispersity index (PI=Mw/Mn) is between 2 and 5 (Otsu et al., 25, 7/8, 643-650, (1989)).
Xanthate disulphides and dithiocarbamate disulphides are themselves well known as transfer agents in conventional radical polymerization in thermal mode and in the presence of an initiator, but no one has hitherto been able to control the polymerization, or even less to produce block copolymers.
Up till now it was known that disulphides (tetraalkylthiuram disulphide, diisopropylxanthate disulphide and mercaptobenzothiazol disulphide) were activatable thermally or under UV irradiation, whereas monosulphides (substituted xanthates, dithiocarbamates) were activatable only under UV irradiation (Roha et al., Macromol. Symp., 91, 81-92, (1995), and Okawara et al., Bull. of the Tokyo Inst. of Techn., No. 78, 1966).
However, controlled radical polymerization making use of a UV irradiation source is very difficult to carry out, especially from an industrial standpoint, since the penetration of the UV photons into the polymerization medium is limited, both by absorption phenomena (most of the ethylenic monomers absorb in the 210-280 nm range) and by diffusion phenomena in disperse media (suspension, emulsion).
Moreover, it has been shown (Turner et al., Macromolecules, 23, 1856-1859, (1990)) that photopolymerization in the presence of dithiocarbamate generates carbon disulphide and may be accompanied by a loss of polymerization control.
For these reasons, it has thus been sought to develop a technique which can be used to obtain block copolymers by a process without UV irradiation, preferably by thermal initiation. Until the present time, no controlled radical polymerization system has been able to be demonstrated using dithiocarbamate compounds in the absence of a UV source.
Document WO 98/01478 describes a process for preparing block polymers by controlled radical polymerization. According to that document, such a process cannot be implemented with the aid of compounds, called chain-transfer agents, chosen from dithiocarbamates, of general formula: 
Controlled radical polymerization has an advantage over conventional radical polymerization when it is a question of preparing low-molecular-weight functionalized chains (reactive telomers). Such polymers are desirable for specific applications such as, for example, coatings and adhesives.
Thus, when it is attempted to synthesize chains grafted with, on average, 2 functional comonomers, the fraction of chains with at most one functional site becomes large when the average degree of polymerization is less than a threshold value (e.g. 20 or 30). Controlled radical polymerization makes it possible to reduce, or even to inhibit, the formation of these oligomers having zero or one functional site which degrade the performance in terms of application.
One object of the present invention is to provide a novel controlled radical polymerization process for the synthesis of block polymers from dithiocarbamates.
Another object of the present invention is to provide a novel controlled radical polymerization process for the synthesis of block polymers from dithiocarbamates in the absence of a UV source.
Another object is to provide a controlled radical polymerization process for the synthesis of block polymers from all types of monomers.
Another object is to provide a controlled radical polymerization process for the synthesis of block polymers containing no metal impurities deleterious to their use.
Another object is to provide a controlled radical polymerization process for the synthesis of block copolymers, the said polymers being chain-end functionalized.
Another object is to provide a controlled radical polymerization process for the synthesis of block polymers and block copolymers having a low polydispersity index.
Another object is to provide a controlled radical polymerization process for the synthesis of oligomers in which the number of functional units is constant from chain to chain.
To these ends, the invention relates to a process for preparing block polymers of general formula (IA) or (IB): 
in which formulae:
R1 represents:
an optionally substituted alkyl, acyl, aryl, alkene or alkyne group (i), or
an optionally substituted or aromatic, saturated or unsaturated, carbocycle (ii), or
an optionally substituted or aromatic, saturated or unsaturated, heterocycle (iii),
it being possible for these groups and rings (i), (ii) and (iii) to be substituted with substituted phenyl groups, substituted aromatic groups, or groups: alkoxycarbonyl or aryloxycarbonyl (xe2x80x94COOR), carboxyl (xe2x80x94COOH), acyloxy (xe2x80x94O2CR), carbamoyl (xe2x80x94CONR2), cyano (xe2x80x94CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (xe2x80x94OH), amino (xe2x80x94NR2), halogen, allyl, epoxy, alkoxy (xe2x80x94OR), S-alkyl, S-aryl, organosilyl, groups having a hydrophilic or ionic character, such as the alkali metal salts of carboxylic acids, the alkali metal salts of sulphonic acid, polyalkylene oxide chains (PEO, PPO), cationic substituents (quaternary ammonium salts),
R representing an alkyl or aryl group,
Z is an optionally substituted ring comprising a nitrogen atom via which Z is linked to the C(xe2x95x90S)xe2x80x94S-group of formula (IA), the other atoms of the said ring inducing a delocalizing or electron-withdrawing effect with respect to the electron density of the nitrogen atom,
R2 and R3, which are identical or different, represent:
an optionally substituted alkyl, acyl, aryl, alkene or alkyne group (i), or
an optionally substituted or aromatic, saturated or unsaturated, carbocycle (ii), or
an optionally substituted, saturated or unsaturated, heterocycle (iii),
it being possible for these groups and rings (i), (ii) and (iii) to be substituted with:
substituted phenyl groups or substituted aromatic groups,
groups: alkoxycarbonyl or aryloxycarbonyl (xe2x80x94COOR), carboxyl (xe2x80x94COOH), acyloxy (xe2x80x94O2CR), carbamoyl (xe2x80x94CONR2) cyano (xe2x80x94CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (xe2x80x94OH), amino (xe2x80x94NR2), halogen, allyl, epoxy, alkoxy (xe2x80x94OR), S-alkyl, S-aryl,
groups having a hydrophilic or ionic character, such as the alkali metal salts of carboxylic acids, the alkali metal salts of sulphonic acid, polyalkylene oxide chains (PEO, PPO), cationic substituents (quaternary ammonium salts),
R representing an alkyl or aryl group,
and, for at least R2 or R3, these groups and rings (i), (ii) and (iii) induce a delocalizing or electron-withdrawing effect with respect to the electron density of the nitrogen atom to which R2 and R3 are linked,
V, Vxe2x80x2, W and Wxe2x80x2, which are identical or different, represent: H, an alkyl group or a halogen,
X, Xxe2x80x2, Y and Yxe2x80x2, which are identical or different, represent H, a halogen or a group chosen from Rxe2x80x2, ORxe2x80x2, OCORxe2x80x2, NHCOH, OH, NH2, NHRxe2x80x2, N(Rxe2x80x2)2, (Rxe2x80x2)2N+Oxe2x88x92, NHCORxe2x80x2, CO2H, CO2Rxe2x80x2, CN, CONH2, CONHRxe2x80x2or CONRxe2x80x22, in which Rxe2x80x2is chosen from alkyl, aryl, aralkyl, alkaryl, alkene or organosilyl groups, optionally perfluorinated and optionally substituted with one or more carboxyl, epoxy, hydroxyl, alkoxy, amino, halogen or sulphonic groups,
a and b, which are identical or different, are equal to 0 or 1,
m and n, which are identical or different, are greater than or equal to 1 and, when one or other is greater than 1, the individual repeat units are identical or different,
in which process the following are brought into contact with each other:
an ethylenically unsaturated monomer of formula:
CYYxe2x80x2(xe2x95x90CWxe2x80x94CWxe2x80x2)axe2x95x90CH2,
a precursor compound of general formula (IIA) or (IIB): 
in which Z, X, Xxe2x80x2, V, Vxe2x80x2, R1, R2 and R3 have the same meaning, and b and n the same value, as previously,
a radical polymerization initiator.
The process therefore consists in bringing into contact with each other a radical polymerization initiator, an ethylenically unsaturated monomer and a precursor of general formula (IIA) or (IIB).
The radical polymerization initiator may be chosen from the initiators conventionally used in radical polymerization. These may, for example, be one of the following initiators:
hydrogen peroxides such as: tert-butyl hydroperoxide, cumene hydroperoxide, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxyoctoate, tert-butyl peroxyneodecanoate, tert-butyl peroxyisobutyrate, lauroyl peroxide, tert-amyl peroxypivalate, tert-butyl peroxypivalate, dicumyl peroxide, benzoyl peroxide, potassium persulphate and ammonium persulphate;
azo compounds such as: 2-2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(2-butanenitrile), 4,4xe2x80x2-azobis(4-pentanoic acid), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), 2-(tert-butylazo)-2-cyanopropane, 2,2xe2x80x2-azobis(2-methyl-N-(1,1)-bis(hydroxymethyl)-2-hydroxyethyllpropionamide, 2,2xe2x80x2-azobis(2-methyl-N-hydroxyethyl]propionamide, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisdbutyramidine)dichloride, 2,2xe2x80x2-azobis(2-amidinopropane)dichloride, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramide), 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide), 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide), 2,2xe2x80x2-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and 2,2xe2x80x2-azobis(isobutyramide)dihydrate;
redox systems including combinations such as:
mixtures of hydrogen peroxide or alkyl peroxide, peresters, percarbonates and the like and of any one of the salts of iron, titanous salts, zinc formaldehyde sulphoxylate or sodium formaldehyde sulphoxylate, and reducing sugars;
alkali-metal or ammonium persulphates, perborates or perchlorates in combination with an alkali metal bisulphite, such as sodium metabisulphite, and reducing sugars;
alkali-metal persulphates in combination with an arylphosphinic acid, such as benzenephosphonic acid and other similar acids, and reducing sugars.
Preferably, the amount of initiator to be used is determined so that the amount of radicals generated is at most 25 mol % with respect to the amount of compound (IIA) or (IIB), even more preferably at most 15 mol %.
As ethylanically unsaturated monomer, the monomers chosen from styrene or its derivatives, butadiene, chloroprene, (meth)acrylic esters, vinyl esters and vinyl nitrites are more specifically used according to the invention.
Butadiene and chloroprene correspond to the case in which a and b=1 in the formulae (IA), (IB), (IIA) or (IIB) and in the formula for the monomer given above.
xe2x80x9c(Meth)acrylic estersxe2x80x9d should be understood to mean esters of acrylic acid and of methacrylic acid with hydrogenated or fluorinated C1-C12, preferably C1-C8, alcohols. Among compounds of this type, mention may be made of: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, tert-butyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate and isobutyl methacrylate.
The vinyl nitrites include more particularly those having from 3 to 12 carbon atoms, such as, in particular, acrylonitrile and methacrylonitrile.
It should be noted that styrene may be replaced, completely or partly, by derivatives such as alpha-methylstyrene or vinyltoluene.
The other ethylenically unsaturated monomers which can be used, alone or as mixtures, or which can be copolymerized with the above monomers, are, in particular:
vinyl esters of carboxylic acid, such as vinyl acetate, vinyl Versatate(copyright) and vinyl propionate;
vinyl halides;
ethylenically unsaturated monocarboxylic and dicarboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid, and monoalkyl esters of dicarboxylic acids of the type mentioned with alkanols preferably having from 1 to 4 carbon atoms and their N-substituted derivatives;
amides of unsaturated carboxylic acids, such as acrylamide, methacrylamide, N-methylolacrylamide or methacrylamide, and N-alkylacrylamides;
ethylenic monomers containing a sulphonic acid group and its ammonium or alkali metal salts, for example vinylsulphonic acid, vinylbenzenesulphonic acid, xcex1-acrylamidomethylpropanesulphonic acid and 2-sulphoethylene methacrylate;
amides of vinylamine, especially vinylformamide or vinylacetamide; and
unsaturated ethylenic monomers containing a secondary, tertiary or quaternary amino group, or a heterocyclic group containing nitrogen, such as, for example, vinylpyridines, vinylimidazole, aminoalkyl (meth)acrylates and aminoalkyl (meth)acrylamides such as dimethylaminoethyl (meth)acrylate, di-tert-butylaminoethyl (meth)acrylate and dimethylamino(meth)acrylamide. Likewise, it is possible to use zwitterionic monomers such as, for example, sulphopropyl(dimethyl)aminopropyl acrylate.
In order to prepare the copolymers of formula (IA) or (IB) for which Y=H and Yxe2x80x2=NH2, it is preferred to use as ethylenically unsaturated monomers the amides of vinylamine, for example vinylformamide or vinylacetamide. The copolymer obtained is then hydrolysed to acid or basic pH.
In order to prepare the copolymers of formula (IA) or (IB) for which Y=H and Yxe2x80x2=OH, it is preferred to use as ethylenically unsaturated monomers vinyl esters of carboxylic acid such as, for example, vinyl acetate. The copolymer obtained is then hydrolysed to acid or basic pH.
The types and amounts of copolymerizable monomers employed according to the present invention vary depending on the particular final application for which the block polymer is intended. These variations are well known and may be easily determined by those skilled in the art.
In order for the polymer of general formula (IA) or (IB) to be a block polymer, the xe2x80x9cprecursorxe2x80x9d co und of general formula (IIA) or (IIB) must be a polymer. Thus, n is greater than or equal to 1, preferably greater than 5. The monomer units of this polymer may be identical or different.
The essential characteristic of the invention stems from the nature of this precursor of general formula (IIA) or (IIB). This precursor (IIA) or (IIB) forms part of the family of dithiocarbamates, the functional group of which is: 
In the case of copolymers of formula (IA) or of precursor polymers of formula (IIA), the nitrogen atom of the dithiocarbamate functional group must form part of a ring and the other atoms of the said ring must exhibit an electron-withdrawing effect on the doublet of the nitrogen of the dithiocarbamate functional group.
The nature of this ring Z, comprising the nitrogen of the dithiocarbamate functional group, can vary, given that there is an electron-withdrawing effect on the doublet of the nitrogen.
On account of the process, the multiblock polymers of formula (IA) have the same characteristics with regard to the ring Z.
In compounds of formula (IA) and (IIA), the ring Z is a ring based on carbon atoms.
This carbocycle may include at least one heteroatom other than the nitrogen which links the ring to xe2x80x94C(xe2x95x90S)xe2x80x94Sxe2x80x94; this heteroatom may be chosen from O, S, N and/or P. Preferably it is O or N.
The ring Z may be an aromatic or heteroaromatic ring.
The ring Z may be functionalized and comprise at least one of the following functional groups: carbonyl (Cxe2x95x90O). SO2, PORxe2x80x3, Rxe2x80x3 representing an alkyl, aryl, OR, SR or NR2 group, where R represents an alkyl or aryl group, these being identical or different. Preferably, the functionalized group is carbonyl.
It is also preferable for the functionalized group to be directly linked to the nitrogen of the dithiocarbamate functional group.
The ring Z may be substituted with at least one of the following groups: alkyl, aryl, alkoxycarbonyl or aryloxycarbonyl (xe2x80x94COOR), carboxyl (xe2x80x94COOH), acyloxy (xe2x80x94O2CR), carbamoyl (xe2x80x94CONR2), cyano (xe2x80x94CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (xe2x80x94OH), amino (xe2x80x94NR2), halogen, allyl, epoxy, alkoxy (xe2x80x94OR), S-alkyl, S-aryl, groups having a hydrophilic or ionic character, such as the alkali metal salts of carboxylic acids or the alkali metal salts of sulphonic acid, polyalkylene oxide chains (PEO, PPO), cationic substituents (quaternary ammonium salts), R representing an alkyl or aryl group.
The ring Z may also be substituted with at least one carbocycle or a heterocycle; this being optionally aromatic and/or substituted with one of the preceding groups. In the latter case, and according to a preferred variant, the ring Z and its cyclic substituent have two common atoms.
The ring Z is preferably chosen from one of the following rings: 
Likewise, in the case of copolymers of formila (IB) or of precursor polymers of formula (IIB), the nitrogen atom of the dithiocarbamate functional group must be linked to R2 and R3 groups, at least one of which induces a delocalizing or electron-withdrawing effect with respect to the electron density of the nitrogen atom of the dithiocarbamate functional group.
According to a first variant, R2 and/or R3 exert a xcfx80 withdrawing effect. For this purpose, R2 and/or R3 may represent a carbonyl or (hetero)aromatic group.
According to a second variant, R2 and/or R3 exert a xcexa3 withdrawing effect. For this purpose, R2 and/or R3 may represent an alkyl group substituted with electron-withdrawing groups.
With regard to the substituent R1 of the compounds of formula (IA), (IB), (IIA) and (IIB), it preferably represents:
a group of formula CR11R12R13, in which:
R11, R12 and R13 represent groups (i), (ii) or (iii) as defined above, or
R11=R12=H and R13 is an aryl, alkene or alkyne group,
or a xe2x80x94COR14 group in which R14 represents a group (i), (ii) or (iii) as defined above.
It may especially be chosen from the following groups: 
The precursor polymer of formula (IIA) may come from the radical polymerization of an ethylenically unsaturated monomer of formula: CXXxe2x80x2(xe2x95x90CVxe2x80x94CVxe2x80x2)bxe2x95x90CH2 by bringing the said monomer into contact with a radical polymerization initiator and a compound of general formula (IIIA) or (IVA): 
p being between 2 and 10, preferably between 2 and 5.
In the general formulae (IIIA) or (IVA), the symbols Z and R1 have the same meaning as previously. The preferences with regard to its symbols are the same as above.
Among compounds of formula (IVA), when p=2, R1 may be chosen from the groups xe2x80x94CH2-phenyl-CH2xe2x80x94 or xe2x80x94(CH2)qxe2x80x94, where q is between 2 and 10.
According to the preferred variants, the compound of formula (IIIA) is chosen from those of the following formulae (A) to (E): 
Likewise, the precursor compound of general formula (IIB) may come from the radical polymerization of an ethylenically unsaturated monomer of formula: CXXxe2x80x2(xe2x95x90CVxe2x80x94CVxe2x80x2)bxe2x95x90CH2 during which the said monomer is brought into contact with a radical polymerization initiator and a compound of general formula (IIIB), (IVB) or (VB): 
p being between 2 and 10.
In the general formulae (IIIB), (IVB) or (VB), the symbols R1, R2 and R3 have the same meaning as previously. The preferences with regard to its symbols are the same as above.
According to the preferred variants, the compound of formula (IIIB) is chosen from the compounds of the following formulae: 
The compounds of formula (IIIA) or (IIIB) are generally obtained by the reaction of the corresponding amine with CS2 so as to obtain the salts of formulae: 
in which M represents sodium, potassium or lithium.
This salt is then brought into contact with a halogen-containing derivative Hal-R1 (Hal represents Cl, Br or I) in order to give the precursor of formula (IIIA) or (IIIB).
During the synthesis of the precursor polymer of formula (IIA) or (IIB), the radical polymerization initiators and the ethylenically unsaturated monomers are of the type of those mentioned previously.
The complete process of synthesizing a block polymer of formula (IA) or (IB) according to the invention may therefore consist in:
(1) synthesizing a polymer by bringing into contact with each other an ethylenically unsaturated monomer of formula: CXXxe2x80x2(xe2x95x90CVxe2x80x94CVxe2x80x2)bxe2x95x90CH2, a radical polymerization initiator and a compound of formula (IIIA), (IIIB), (IVA), (IVB) or (VB), and
(2) using this polymer obtained at step (1) as precursor of general formula (IIA) or (IIB) in order to prepare a diblock polymer by bringing it into contact with a new ethylenically unsaturated monomer of formula: CYYxe2x80x2(xe2x95x90CWxe2x80x94CWxe2x80x2)bxe2x95x90CH2 and a radical polymerization initiator.
This step (2) may be repeated as many times as desired using new monomers to synthesize new blocks and to obtain a multiblock polymer.
As indicated previously, for the preparation of precursors of formula (IIA) or (IIB) for which X=H and Xxe2x80x2=NH2, it is preferred to use, as ethylenically unsaturated monomers, amides of vinylamine, for example vinylformamide or vinylacetamide. The polymer obtained is then hydrolysed to acid or basic pH.
Likewise, for the preparation of precursors of formula (IIA) or (IIB) for which X=H and Xxe2x80x2=OH, it is preferred to use vinyl esters of carboxylic acids, such as vinyl acetate for example, as ethylenically unsaturated monomers. The polymer obtained is then hydrolysed to acid or basic pH.
According to this principle, the invention therefore also relates to a process for preparing multiblock polymers, in which the implementation of the process previously described is repeated at least once, using:
different monomers from those of the evious implementation, and
instead of the precursor compound of rmula (IIA) or (IIB), the block polymer coming from the previous implementation.
If the implementation is repeated once, a triblock polymer will be obtained, if it is repeated twice, a xe2x80x9cquadriblockxe2x80x9d polymer will be obtained, and so on. In this way, at each new implementation, the product obtained is a block polymer having an additional polymer block.
Therefore, in order to prepare multiblock polymers, the process consists in repeating, several times, the implementation of the preceding process on the block polymer coming from each previous implementation using different monomers.
According to this method of preparing multiblock polymers, when it is desired to obtain homogeneous block polymers without a composition gradient, and if all the successive polymerizations are carried out in the same reactor, it is essential for all the monomers used in one step to have been consumed before the polymerization of the next step starts, therefore before the new monomers are introduced.
The compounds of formula (IVA) and (IVB) are particularly advantageous as they allow a polymer chain to be grown on at least two active sites. With this type of compound, it is possible to save on polymerization steps in order to obtain an n-block copolymer.
Thus, if p=2, the first block is obtained by the polymerization of a monomer M1 in the presence of the compound of formula (IVA) or (IVB). This first block may then grow at each of its ends by the polymerization of a second monomer M2. A triblock copolymer is obtained. This triblock polymer itself may grow at each of its ends by the polymerization of a third monomer M3. Thus, a xe2x80x9cpentablockxe2x80x9d copolymer is obtained in only three steps.
If p is greater than 2, the process makes it possible to obtain homopolymers or block copolymers whose structure is xe2x80x9cmulti-branchedxe2x80x9d or hyperbranched.
The polymerization is carried out according to any method known to those skilled in the art. It may be carried out in bulk, in solution or in emulsion. The temperature may vary between ambient temperature and 150xc2x0 C., depending on the nature of the monomers used. The process is carried out in the absence of a UV source.
The process according to the invention has the advantage of resulting in block polymers having a low polydispersity index.
It also makes it possible to control the molecular mass of the polymers.
The invention therefore also relates to the block polymers which can be obtained by the above process.
In general, these polymers have a polydispersity index (PI) of at most 2, preferably of at most 1.5.
The preferred block polymers are those having at least two polymer blocks chosen from the following combinations:
polystyrene/polymethyl acrylate,
polystyrene/polyethyl acrylate,
polystyrene/poly(tert-butyl acrylate),
polyethyl acrylate/polyvinyl acetate,
polybutyl acrylate/polyvinyl acetate,
poly(tert-butyl acrylate)/polyvinyl acetate.
Finally, the process for synthesizing the precursor polymers of general formula (IIA) or (IIB) also makes it possible to synthesize polymers having a low polydispersity index. In general, these precursor polymers have a polydispersity index of at most 2, preferably of at most 1.5.
Preferably, for these precursor polymers of general formula (IIA) or (IIB), n is greater than or equal to 6.
The following examples illustrate the invention without, however, limiting the scope thereof.